1. Field of the Invention
The present invention relates to a projection display apparatus equipped with reflective liquid crystal (LC) display devices and a three-plate type of liquid crystal projector that uses the projection display apparatus as for modulating each monochromatic light.
2. Description of the Related Art
In recent years, liquid crystal projectors have been incorporated into popular tools that can be used as large-sized screen display apparatuses. Such display apparatuses, which can be used for various things, such as meetings with screen presentation, home theaters, and other uses, have been developed on various types of display techniques.
In particular, a projection display apparatus which processes respective monochromatic light channels for the primary colors adopts reflective liquid crystal display devices.
Each reflective liquid crystal device has two substrates and a liquid crystal layer, where one substrate has a surface with a transparent electrode formed thereon and the other substrate has a surface on which reflective electrodes and drive circuits for respective pixels are arranged in a matrix and both surfaces of the two substrates are opposed in parallel to each other to sandwich the liquid crystal therebetween as a layer. In this display device, the drive circuits for the liquid crystal can be embedded beneath the display pixels, thus allowing the projection display apparatus to present images with high spatial resolution and high brightness.
The liquid crystal display devices utilize the double refraction of liquid crystal molecules to control the transmission of the light passing therethrough. Thus how to orient the liquid crystal molecules has a large influence on the display quality of the images. In such situations, a vertically aligned type of liquid crystal (on homeotropic aligned mode) has attracted attentions, because this type of liquid crystal provides high contrast and has a faster response time than a horizontally aligned type of liquid crystal (on homogeneous aligned mode).
In the horizontally aligned type of liquid crystal, the liquid crystal molecules are aligned almost in parallel with the substrate plates when no voltage is applied between the substrates, while the molecules are aligned perpendicularly to the substrate surfaces thanks to their dielectric anisotropy in response to application of the voltage therebetween, whereby the black-and-white display can be realized. However, in the perpendicularly aligned state, some molecules closely positioned to alignment membranes on the respective substrates are still kept at alignment angles which are close to the horizon, thus deteriorating a black level due to retardation to be caused (i.e., the contrast is obliged to be lowered). By contrast, the vertically aligned type of liquid crystal has a negative dielectric anisotropy, so that even no application of voltage between the substrates permits the liquid crystal molecules to be aligned almost perpendicularly to the substrates, whilst applying the voltage therebetween allows the molecules to be aligned in parallel with the substrates. This makes the contrast high and makes it possible to operate the molecules faster with less driving power.
There is one thing, however, that should be noted when the vertically aligned type of liquid crystal is used. Namely, this will be caused when the voltage is applied in a state where the alignment is completed such that the liquid crystal molecules are completely perpendicular to the substrate surfaces in response to the voltage non-application. That is, the liquid crystal molecules fall down in different various directions by responding to the voltage application, bringing about disclination which deteriorates image quality. To suppress this drawback, as shown in FIG. 11, the vertically aligned type of liquid crystal is subjected to have a pre-tilt angle θp to allow the molecules to keep a slight inclination in a predetermined direction during non-application of the voltage between the substrates. In FIG. 13, an angle ψ is an azimuthal angle made between a predetermined axis direction on the substrate surface(s) and a projected direction of the major axis of the liquid crystal molecule onto the substrate surface(s).
It is also known that, in the projection display apparatus, the reflective liquid crystal display device can be equipped with not only the reflective liquid crystal display device including the vertically aligned type of liquid crystal but also a wire grid type of polarizing beam splitter (hereinafter referred to as “WG-PBS”) serving as an incident and reflecting optical system for polarized light entering or coming out of the display device. This structure is able to provide projection images with very high contrast (refer to for example United States Patent Laid-open Publication No. 2003/0128320). In a reflective liquid crystal display device shown in this publication, alignment membranes are placed on surfaces of both a transparent substrate and an active matrix substrate so as to be exposed to liquid crystal sandwiched between the substrates as a layer. Using those membranes, the molecules of the liquid crystal layer are given the alignment conditions (i.e., pre-tilt angle θp and azimuthal angle ψ) illustrated in FIG. 13.
In this alignment, the pre-tilt angle θp is given to the liquid crystal molecules, thus causing retardation more or less in the plane direction of the liquid crystal layer, decreasing the contrast.
To avoid this drawback, a countermeasure is known which is to employ a phase compensator (e.g., phase compensating plate) to compensate for differences in the phase of reflected light. This compensator is placed on the polarized-light incident and outgoing side of a reflective liquid crystal display device. United States Patent Laid-open Publication No. 2003/0164909 discloses the relationship between angles of a late-phase axis of a phase compensator and the azimuthal angles of liquid crystal molecules.
On the other hand, the foregoing projection display apparatus with the foregoing configuration can be applied to a single-color light processor, as one of the three processors for the three primary colors, of a three-plate type of liquid crystal projector. With regard to this projector, various proposals, such as U.S. Pat. No. 6,857,747 and Japanese Patent Laid-open Publication No. 2002-098937, have known.
As stated so far, it has been known that the projection display apparatus is able to provide images of higher contrast if the apparatus is produced by using not merely the reflective liquid crystal display device with the vertically aligned type of liquid crystal but also the WG-PBS to be combined with the display device. However, it is admitted to the inventors that alignment conditions for liquid crystal molecules have yet to be disclosed by anybody. In particular, it is absolutely necessary for the projection display apparatus that the bright state (the liquid crystal molecules are flipped down due to a voltage application) outputs a high brightness and the dark state outputs the lowest brightness to the best of apparatus's ability. If such bright and dark states are produced, high contrast can be given to images. However, optimum alignment conditions for producing those two states have been unknown yet.